In general, a load cell monitoring torsional force includes as elements formed in a resilient body opposing; load-receiving ends spaced along an axis of rotation and a number of sensing beams aligned with the axis extending between the ends. The sensing beams have strain gages applied thereto. The torque applied to the structure results in torsional forces producing dimensional changes in the sensing beams. The electrical strain gages are sensitive to the changes and an accurate reading of the applied load is derived by external measurement apparatus.
One of the many advantages of a load cell is the lack of individual mechanical parts that are responsive to applied loads and therefore subject to wear from repeated use. The rendering of repeatable and accurate readings by load cells relies primarily on the structural integrity of the design and the materials used in the load cell structure. Thus, a unitary design relying on a single structural member provides advantages. The application of forces in excess of the rated capacity can alter the geometry of the load cell structure thereby introducing errors into subsequent readings. Both accuracy and precision can be adversely affected by loading in excess of ratings.
To compensate for the application of excessive loading of the structure, overload protection is often incorporated into the load. The provision of overload protection in load cells used in the measurement of linear or non-rotational forces is discussed in U.S. Pat. No. 6,755,087 to Clegg wherein a cantilever beam bounded by two sensing beams is provided in a unitary structure. The linear forces are applied to the opposing ends of the structure thereby causing the sensing beams to deflect. By the use of narrow slots machined into the load cell structure, excess loading results in contact of the sensing beams with the cantilever beam to transfer loads thereto and thus provides overload protection for the structure.
The torsion monitoring load cell, unlike conventional linear force measuring load cells, monitors torque occurring about an axis of rotation. As a result, the elements of a unitary structure are both aligned with the axis of rotation and disposed therearound. The incorporation of overload protection into a torsion monitoring load cell of unitary structures is not readily accomplished. Attempts have been made to provide independent mechanical stops in order to limit relative rotations of the force-receiving ends. The use of mechanical stops is not favored for they are difficult to set, often engage early giving rise to false readings and fail to provide reliable protection. The omission of overload protection is often compensated for by designing a load cell tolerating high levels of sensing beam distortion. The materials in the high deflection load cells exhibit less stiffness and are less responsive to changing loads. Frequency response is important in many torsional load cell applications.
Accordingly, the present invention is directed to a load cell structure for monitoring torsional forces having overload protection incorporated into a single unitary structure without requiring the use of additional parts. The structure provides bidirectional protection of the load cell against both sudden impact forces and torque in excess of rated capacity. The unique constructional features of the present load cell facilitate the manufacture thereof while providing integral overload protection.